Oxygen getters for anode protection in a solid-oxide fuel cell stack

ABSTRACT

In a fuel cell assembly, nickel-based anodes are readily oxidized when exposed to oxygen as may happen through atmospheric invasion of the assembly during cool-down following shutdown of the assembly. Repeated anode oxidation and reduction can be destructive of the anodes, leading to cracking and failure. To prevent such oxygen migration, oxygen getter devices containing oxygen-gettering material such as metallic nickel are provided in the fuel passageways leading to and from the anodes. Oxidation of the oxygen-gettering material is readily reversed through reduction by fuel when the assembly is restarted.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.10/178,826, which was filed on Jun. 24, 2002.

TECHNICAL FIELD

The present invention relates to hydrogen/oxygen fuel cells having asolid-oxide electrolytic layer separating an anode layer from a cathodelayer; more particularly, to fuel cell stack assemblies and systemscomprising a nickel-based anode; and most particularly, to such fuelcell assemblies and systems wherein the anode is protected fromoxidation, especially during cool-down after the assembly has been shutdown.

BACKGROUND OF THE INVENTION

Fuel cells which generate electric current by the electrochemicalcombination of hydrogen and oxygen are well known. In one form of such afuel cell, an anodic layer and a cathodic layer are separated by anelectrolyte formed of a ceramic solid oxide. Such a fuel cell is knownin the art as a “solid oxide fuel cell” (SOFC). Hydrogen, either pure orreformed from hydrocarbons, is flowed along the outer surface of theanode and diffuses into the anode. Oxygen, typically from air, is flowedalong the outer surface of the cathode and diffuses into the cathode.Each O₂ molecule is split and reduced to two O⁻² anions catalytically bythe cathode. The oxygen anions transport through the electrolyte andcombine at the anode/electrolyte interface with four hydrogen ions toform two molecules of water. The anode and the cathode are connectedexternally through a load to complete the circuit whereby four electronsare transferred from the anode to the cathode. When hydrogen is derivedfrom “reformed” hydrocarbons, the “reformate” gas includes CO which isconverted to CO₂ at the anode via an oxidation process similar to thatperformed on the hydrogen. Reformed gasoline is a commonly used fuel inautomotive fuel cell applications.

A single cell is capable of generating a relatively small voltage andwattage, typically between about 0.5 volt and about 1.0 volt, dependingupon load, and less than about 2 watts per cm² of cell surface.Therefore, in practice it is usual to stack together, in electricalseries, a plurality of cells. Because each anode and cathode must have afree space for passage of gas over its surface, the cells are separatedby perimeter spacers which are vented to permit flow of gas to theanodes and cathodes as desired but which form seals on their axialsurfaces to prevent gas leakage from the sides of the stack. Theperimeter spacers include dielectric layers to insulate theinterconnects from each other. Adjacent cells are connected electricallyby “interconnect” elements in the stack, the outer surfaces of theanodes and cathodes being electrically connected to their respectiveinterconnects by electrical contacts disposed within the gas-flow space,typically by a metallic foam which is readily gas-permeable or byconductive filaments. The outermost, or end, interconnects of the stackdefine electric terminals, or “current collectors,” which may beconnected across a load.

A complete SOFC system typically includes auxiliary subsystems for,among other requirements, generating fuel by reforming hydrocarbons;tempering the reformate fuel and air entering the stack; providing airto the hydrocarbon reformer; providing air to the cathodes for reactionwith hydrogen in the fuel cell stack; providing air for cooling the fuelcell stack; providing combustion air to an afterburner for unspent fuelexiting the stack; and providing cooling air to the afterburner and thestack. A complete SOFC assembly also includes appropriate piping andvalving, as well as a programmable electronic control unit (ECU) formanaging the activities of the subsystems simultaneously.

The anodes of cells in a fuel cell assembly typically include metallicnickel and/or a nickel cermet (Ni-YSZ) which are readily oxidized.During operation of an assembly, the anodes are in a reduced state. Aproblem exists in that the anodes are vulnerable to oxidation byatmospheric oxygen which can enter the stacks via the reformatepassageways during cool-down of the assembly. The grain growth andcontraction of the metallic nickel in the anode during theoxidation/reduction cycle is not readily managed by the ceramiccomponent. Repeated oxidation and reduction of nickel in the anodes canlead to unwanted structural changes and can result in catastrophiccracking of the anodes.

It is a principal object of the present invention to protect the nickelanodes of a fuel cell from structural degradation by periodic oxidationand reduction of the nickel.

It is a further object of the present invention, through suchprevention, to improve the reliability and extend the lifetime of solidoxide fuel cells.

BRIEF DESCRIPTION OF THE INVENTION

Briefly described, in a fuel cell assembly, for example, a solid-oxidefuel cell assembly, metallic nickel in a Ni-YSZ based anode is readilyoxidized when exposed to oxygen as may happen through atmosphericinvasion of the assembly during cool-down following shutdown of theassembly. Anodes are in an oxidized equilibrium state when the assemblyis fabricated and are then reduced by reformate when the assembly isfirst turned on. Repeated anode oxidation and reduction can bedestructive of the three-dimensional structure of the anodes and canlead to cracking and failure of the anodes and thus the entire assembly.To prevent such oxygen migration and re-oxidation, oxygen getterdevices, themselves containing oxygen-scavenging material such asmetallic nickel, are provided in the reformate passageways leading toand from the anodes. When oxidized, the oxygen-gettering material isreadily reversed through reduction by reformate when the assembly isrestarted.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the invention will be morefully understood and appreciated from the following description ofcertain exemplary embodiments of the invention taken together with theaccompanying drawings, in which:

FIG. 1 is a schematic cross-sectional view of a two-cell stack of solidoxide fuel cells;

FIG. 2 is a schematic elevational view of two fuel cell stackselectrically connected in series; and

FIG. 3 is a schematic mechanization diagram of an SOFC assembly, showingthe incorporation of oxygen getters in fuel passageways leading into andout of the anodes.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, a fuel cell stack 10 includes elements known in theart of solid oxide fuel cell stacks comprising more than one fuel cell.The example shown includes two identical fuel cells 11, connected inseries, and is of a class of such fuel cells said to be“anode-supported” in that the anode is a structural element having theelectrolyte and cathode deposited upon it. Element thicknesses as shownare not to scale.

Each fuel cell 11 includes an electrolyte element 14 separating ananodic element 16 and a cathodic element 18. Each anode and cathode isin direct chemical contact with its respective surface of theelectrolyte, and each anode and cathode has a respective free surface20,22 forming one wall of a respective passageway 24,26 for flow of gasacross the surface. Anode 16 of a first fuel cell 11 faces and iselectrically connected to an interconnect 28 by filaments 30 extendingacross but not blocking passageway 24. Similarly, cathode 18 of a secondfuel cell 11 faces and is electrically connected to interconnect 28 byfilaments 30 extending across but not blocking passageway 26. Similarly,cathode 18 faces and is electrically connected to a cathodic currentcollector 32 by filaments 30 extending across but not blockingpassageway 26, and anode 16 faces and is electrically connected to ananodic current collector 34 by filaments 30 extending across but notblocking passageway 24. Current collectors 32,34 may be connected acrossa load 35 in order that the fuel cell stack 10 performs electrical work.Passageways 24 are formed by anode spacers 36 between the perimeter ofanode 16 and either interconnect 28 or anodic current collector 34.Passageways 26 are formed by cathode spacers 38 between the perimeter ofelectrolyte 14 and either interconnect 28 or cathodic current collector32. Anode spacer 36 and cathode spacer 38 are formed from sheet stock insuch a way to yield the desired height of the anode passageways 24 andcathode passageways 26.

Preferably, the interconnect and the current collectors are formed of analloy, typically a “superalloy,” which is chemically and dimensionallystable at the elevated temperatures necessary for fuel cell operation,generally about 750° C. or higher, for example, Hastelloy, Haynes 230,or a stainless steel. The electrolyte is formed of a ceramic oxide andpreferably includes zirconia stabilized with yttrium oxide (yttria),known in the art as YSZ. The cathode is formed of, for example, porouslanthanum strontium manganate or lanthanum strontium iron, and the anodeis formed of, for example, a mixture of nickel and YSZ.

In operation (FIG. 1), reformate gas 21 is provided to passageways 24 ata first edge 25 of the anode free surface 20, flows parallel to thesurface of the anode across the anode in a first direction, and isremoved at a second and opposite edge 29 of anode surface 20. Hydrogenand CO diffuse into the anode to the interface with the electrolyte.Oxygen 31, typically in air, is provided to passageways 26 at a firstedge 39 of the cathode free surface 22, flows parallel to the surface ofthe cathode in a second direction which can be orthogonal to the firstdirection of the reformate (second direction shown in the same directionas the first for clarity in FIG. 1), and is removed at a second andopposite edge 43 of cathode surface 22. Molecular oxygen gas (O₂)diffuses into the cathode and is catalytically reduced to two O⁻² ionsby accepting four electrons from the cathode and the cathodic currentcollector 32 or the interconnect 28 via filaments 30. The electrolyteionically conducts or transports O⁻² anions to the anode electrolyteinnerface where they combine with four hydrogen atoms to form two watermolecules, giving up four electrons to the anode and the anodic currentcollector 34 or the interconnect 28 via filaments 30. Thus cells 11 areconnected in series electrically between the two current collectors, andthe total voltage and wattage between the current collectors is the sumof the voltage and wattage of the individual cells in a fuel cell stack.

Referring to FIG. 2, the cells 11 are arranged side-by-side rather thanin overlapping arrangement as shown in FIG. 1. Further, the side-by-sidearrangement may comprise a plurality of cells 11, respectively, suchthat each of first stack 44 and second stack 46 shown in FIG. 2 is astack of identical fuel cells 11. The cells 11 in stack 44 and stack 46are connected electrically in series by interconnect 47, and the stacksare connected in series.

Referring to FIG. 3, the diagram of a solid-oxide fuel cell assembly 12includes auxiliary equipment and controls for stacks 44,46 electricallyconnected as in FIG. 2.

A conventional high speed inlet air pump 48 draws inlet air 50 throughan air filter 52, past a first MAF sensor 54, through a sonic silencer56, and a cooling shroud 58 surrounding pump 48.

Air output 60 from pump 48, at a pressure sensed by pressure sensor 61,is first split into branched conduits between a feed 62 and a feed 72.Feed 62 goes as burner cooling air 64 to a stack afterburner or tail gascombustor 66 via a second MAF sensor 68 and a burner cool air controlvalve 70.

Feed 72 is further split into branched conduits between an anode airfeed 74 and a cathode air feed 75. Anode feed 74 goes to a hydrocarbonfuel vaporizer 76 via a third MAF sensor 78 and reformer air controlvalve 80. A portion of anode air feed 74 may be controllably diverted bycontrol valve 82 through the cool side 83 of reformate pre-heat heatexchanger 84, then recombined with the non-tempered portion such thatfeed 74 is tempered to a desired temperature on its way to vaporizer 76.

Cathode air feed 75 is controlled by cathode air control valve 86 andmay be controllably diverted through cathode bypass feed 87 by cathodeair preheat bypass valve 88 through the cool side 90 of cathode airpre-heat heat exchanger 92 on its way to stacks 44,46. After passingthrough the cathode sides of the cells in stacks 44,46, the partiallyspent, heated air 93 is fed to afterburner 66.

A hydrocarbon fuel feed pump 94 draws fuel from a storage tank 96 anddelivers the fuel via a pressure regulator 98 and filter 100 to a fuelinjector 102 which injects the fuel into vaporizer 76. The injected fuelis combined with air feed 74, vaporized, and fed to a reformer catalyst104 in main fuel reformer 106 which reforms the fuel to, principally,hydrogen and carbon monoxide. Reformate 108 from catalyst 104 is fed tothe anodes in stacks 44,46. Unconsumed fuel 110 from the anodes is fedto afterburner 66 where it is combined with air supplies 64 and 93 andis burned. The hot burner gases 112 are passed through a cleanupcatalyst 114 in main reformer 106. The effluent 115 from catalyst 114 ispassed through the hot sides 116,118 of heat exchangers 84, 92,respectively, to heat the incoming cathode and anode air. Thepartially-cooled effluent 115 is fed to a manifold 120 surroundingstacks 44,46 from whence it is eventually exhausted 122.

Still referring to FIG. 3, a first oxygen getter device 124 is providedin the conduit feeding fuel such as, for example, pure hydrogen orreformate 108 to the anodes (not visible) in stacks 44,46. A second andsubstantially identical oxygen getter device 126 is similarly providedin the conduit feeding spent fuel 110 from the anodes to afterburner 66.As described above, during cool-down of the fuel cell stacks aftershut-down of the assembly, it is important to prevent migration ofoxygen into anode passageways 24 wherein anode surface 20, comprisingmetallic nickel in a ceramic matrix (nickel/YSZ cermet), would besubject to damaging oxidation. Each getter includes a passageway 128having an inlet 130 and an outlet 132 through which fuel is passedduring operation of the fuel cell assembly. Within the passageway is areadily-oxidized material 134 (oxygen-reducing means), for example,nickel metal foam, nickel wire or nickel mesh, which is capable ofgettering oxygen by reaction therewith but which does not present asignificant obstruction to flow of fuel through the passageway. Nickelin the getters reacts with oxygen to produce nickel oxide, NiO, when theassembly is shut down, thus protecting the nickel-containing anodes fromoxidation. When the assembly is turned back on, reformate is againproduced which, in passing through the getters, reduces the NiO back tometallic nickel, allowing the getters to be used repeatedly.

An SOFC assembly 1000 in accordance with the invention is especiallyuseful as an auxiliary power unit (APU) 1000-1 for vehicles 136 on whichthe APU may be mounted as shown in FIG. 3, such as, for example, cars136-1 and trucks 136-2, boats and ships 136-3, and airplanes 136-4,wherein motive power is supplied by a conventional engine and theauxiliary electrical power needs are met by an SOFC assembly.

An SOFC assembly in accordance with the invention is also useful as astationary power plant such as, for example, in a household or forcommercial usage.

While the invention has been described by reference to various specificembodiments, it should be understood that numerous changes may be madewithin the spirit and scope of the inventive concepts described.Accordingly, it is intended that the invention not be limited to thedescribed embodiments, but will have full scope defined by the languageof the following claims.

1. A method of protecting a fuel cell assembly from oxidation,comprising: providing a nickel-containing anode; providing a firstpassageway extending from an outlet of said anode for carrying spentexhaust fuel from the anode; and disposing a first oxygen getter in saidfirst passageway to prevent substantially all migration of gaseousoxygen into said nickel-containing anode during a non-operational stateof the fuel cell assembly to prevent said nickel-containing anode fromoxidation.
 2. A method in accordance with claim 1, further comprising:providing a reformer; providing a second passageway extending betweensaid nickel-containing anode and said reformer for providing fuel tosaid anode; providing a second oxygen getter in said second passagewayfor preventing substantially all migration of gaseous oxygen into saidnickel-containing anode during the non-operational state of the fuelcell assembly to prevent said nickel-containing anode from oxidation. 3.A method in accordance with claim 1 wherein said first oxygen getterincludes metallic nickel.
 4. A method in accordance with claim 3 whereinsaid metallic nickel is in a form selected from the group consisting ofnickel metal foam, nickel wire, and nickel mesh.
 5. A method inaccordance with claim 1 wherein said first oxygen getter includes nickelalloy.
 6. A method in accordance with claim 5 wherein said nickel alloyis in a form selected from the group consisting of nickel alloy foam,nickel alloy wire, and nickel alloy mesh.
 7. A method in accordance withclaim 1 further comprising reductively reversing oxidation of said firstoxygen getter using spent exhaust fuel when the fuel cell stack is in anoperational state.
 8. A method in accordance with claim 1 wherein saidfuel cell assembly is mounted on a vehicle.
 9. A method in accordancewith claim 1 wherein said fuel cell assembly is mounted stationary. 10.A method in accordance with claim 8 wherein said vehicle is selectedfrom the group consisting of car, truck, boat, and airplane.
 11. Amethod in accordance with claim 10 wherein said assembly is an auxiliarypower unit for said vehicle.
 12. A method in accordance with claim 1wherein said nickel-containing anode is included in a solid-oxide fuelcell.